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Marginal compression

G

gibletts

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"The compression readings for

"The compression readings for my 74 135HP are 93,112,105 & 110, I am concerned about the marginal compression on #1 and consider at this point some investigation is required, I would be interested in the opinion of others before tearing down motor, thanks dm."
 
"Hi,

I recently had the com

"Hi,

I recently had the compression checked on my 100hp Johnson, it is a 1979. All four cylinders read anywhere from 105 to 115. I was told you should have at the very least 90. Also, the cylinder compression should be within 10% of each other. Given this, I would say you are ok."
 
"I wondered about that 10% thi

"I wondered about that 10% thing, is it 10% from highest compared to lowest (in that case it looks like we have a problem) or highest and lowest compared to the average (in that case it is just about within the tolerance, for the moment at least)?"
 
"As I understand it, it is 10%

"As I understand it, it is 10% plus or minus within each of the cylinders.

What brought you to do a compression test?

Where you having a problem?"
 
"I had some performance issues

"I had some performance issues on the water, when having a look into it I found I had no \ weak spark on a couple of cyclinders, also while I was at it I checked the compressions as above. I figure theres no point in fixing the ignition problem (which I am sure is the culprit to the performance issues) until I am satisfied with the compression."
 
"Probably not the stator, repa

"Probably not the stator, repaced it last year after it started oozing black custard, the two ignition coils associated with dodgy cyclinders dont look to good and are badly cracked etc, so I will replace them to start with and go from there."
 
"Dave, your number 1 cylinder

"Dave, your number 1 cylinder is a little low compared with the others. With your highest compression being 112, I would be looking for nothing below about 100 - so it is a little under.

Did you test the compression with the engine being "warm"? - it does make a difference....


And I would take your assumption to be correct, that the coils may be the problem. The very fact that they appear cracked is reason enough to replace them."
 
"Thanks for the input Larry &a

"Thanks for the input Larry & Graham. I did test the engine cold, I will replace coils, warm her up and re confirm those readings. However if the difference in compressions does diminish while at temp is it fair to disregard the redings taken while the engine was cold or should they still be investigated?"
 
Those motors were famous for b

Those motors were famous for broken piston rings. So it is easy to do an inspection of the intake side of the pistons by removing the 4 bypass covers. It will only cost pennies for the new gaskets and piece of mind.
 
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